1. Field of the Invention
The present invention relates generally to a system and method for recovering regenerative power in a vehicle, and more particularly to a system and method for recovering regenerative power in a vehicle having an electric machine.
2. Background Art
Pure electric and hybrid electric vehicles (i.e., electric vehicles) generally include an electric machine for providing, at least in part, tractive force to the vehicle drive wheels. In addition, electric vehicles generally include a power source, such as a battery, an ultra capacitor and/or the like, to supply power to the electric machine.
In general, the energy (e.g., fuel) economy of electric vehicles may be improved by recovering (i.e., capturing) the vehicle's kinetic energy during vehicle deceleration (i.e., brake regeneration). More particularly, during deceleration, braking torque (i.e., negative torque) may be provided to the driven wheels by using the electric machine as a generator. Accordingly, the electric machine may generate electric charge (i.e., regenerative power) during vehicle deceleration which may then be stored at the power source.
In rear wheel drive (i.e., RWD) electric vehicles, tractive force may be applied to the rear wheels by coupling the electric machine to the rear wheels. Accordingly, during brake regeneration, the electric machine is only capable of recovering kinetic energy from (i.e., providing braking torque to) the rear wheels. Because a vehicle generally becomes unstable when braking torque is applied solely to the rear wheels, friction brakes are often used to provide additional braking torque to the front vehicle wheels. More particularly, stability may be generally maintained when 30% or less of the total braking torque (e.g., the sum of the braking torques generated by the electric machine and the friction brakes) is applied to the rear wheels and 70% or more of the total braking torque is applied to the front wheels. This 30/70 split dictates that, at most, 30% of the total available regenerative power, corresponding to 30% of the total braking torque, may be recovered during deceleration of a RWD vehicle.
Similarly, a four wheel drive (i.e., 4WD) electric vehicle generally includes an electric machine coupled to the rear vehicle wheels and selectively coupled, for example via a transfer case, to the front vehicle wheels such that the tractive force generated by the electric machine may be applied to all four wheels of the vehicle. However, conventional 4WD electric vehicles generally decouple the front vehicle wheels from the electric machine during vehicle deceleration. Accordingly, conventional 4WD electric vehicles generally behave as RWD electric vehicles during deceleration. It follows then, that conventional 4WD electric vehicles are generally capable of recovering, at most, 30% of the total available regenerative power.
It may be desirable, therefore, to have a system and method for increasing the amount of regenerative power (i.e., electric power generated from the braking torque) recovered during deceleration of a vehicle.